[1]} pointed out that the concentration parameter reflects the density of the Universe when the halo formed. In particular, older halos formed in higher-density environments and tend to have larger concentrations. Several theoretical studies found that FGs assembled half of their mass at earlier epochs than non-fossil ones [2]}, [3]}. In this framework, it is expected that FGs would be located in high concentrated halos. Different numerical models are used in the literature to compute halo concentration in clusters [4]}, [5]}. Thus, observational results can be easily compared with theoretical predictions to test the formation scenario of FGs.
